1. Field of the Invention
The invention relates to adaptively decoding addresses on a computer bus and more particularly to an apparatus and method for positively and subtractively decoding addresses on a computer bus, especially the peripheral component interconnect (PCI) bus, when the decoding is determined based upon computer configuration.
2. Description of the Related Art
Personal computers are constantly evolving to provide the user with the highest performance available at the lowest cost. Performance improvements in the microprocessor and memory systems have resulted in computers so powerful that they are now capable of performing tasks that before could only be performed by large mainframe computers. Technological change is especially exemplified in the area of portable computers where power consumption efficiency is balanced against features, cost, size, weight and performance. This is especially challenging since many computer users desire the portable computer to provide nothing less than what a desktop unit can provide. In this specification, the term "portable computer" is used. broadly here to denote the class of computers powered by battery or solar power. Those familiar with portable computers will recognize labels such as: portable, luggable, laptop, notebook and handheld which are used to designate certain marketing segments of the larger portable computer market.
Many options are available to the computer system designer. While simply designing around the highest performance processor available will go a long way towards providing a high performance product, in todays competitive market it is not enough. The processor must be supported by high performing components including a high performance or mezzanine input/output (I/O) bus. Several standardized I/O buses are available to the system designer including: ISA (Industry Standard Architecture); EISA (Extended Industry Standard Architecture); and PCI (Peripheral Component Interface). Today's computers are typically designed with some combination of the three to provide the user with the performance of PCI and backward compatibility to ISA These three buses are familiar to those skilled in the art.
Design choices also involve certain special features of the computer that distinguish one manufacturer's computer from a competitors. In the portable computer market this is especially challenging since added features can cause increased size and weight. For example, since software can require large amounts of storage, a high capacity hard disk drive is often desirable. However, high capacity disk drives are usually much larger and heavier than desirable for a portable computer. It is also desirable to have the ability to add functionality to the portable computer. However, providing expansion bays compromises the small size.
One known method of providing certain features without sacrificing size and weight is through the use of an expansion base unit. An expansion unit is a non-portable unit that typically operates from AC power and resides on the user's desktop. When the user is working at the desk, the portable computer plugs into the expansion unit to provide the added functionality. For example, the expansion unit may have a network interface unit for connecting to a local area network, a high capacity disk drive, a floppy drive and other peripherals. An additional hard drive may also be resident in the expansion base unit.
The connection between the portable computer and the expansion base unit is typically proprietary since no standard has yet evolved. One known way to couple the portable computer to the expansion base unit is through the existing I/O bus. In a portable computer having a PCI bus and an ISA bus, either bus might be used to connect to the expansion base unit. For it high performance alone, the PCI bus is preferable. Additionally, the PCI bus offers a 32-bit multiplexed address and data bus which also makes the PCI bus desirable because of its small connector size requirements.
The PCI bus defines three physical address spaces: memory, I/O space and configuration space. Address decoding on the PCI bus is distributed; i.e. each device coupled to the PCI bus performs address decoding. The PCI specification defines two styles of address decoding: positive and subtractive. Positive decoding is faster since each PCI device is looking for accesses in the address range(s) the device has been assigned. Subtractive decoding can be implemented by only one device on the PCI bus since the subtractive decoding device accepts all accesses not positively decoded by some other agent. This decode mechanism is slower since it must give all other bus agents a "first right of refusal" on the access before it is claimed by the subtractive decoding device. However, it is very useful for an agent such as a standard expansion bus that must respond to a highly fragmented address space. More details on the PCI bus and particularly PCI addressing are found in the PCI Local Bus Specification, Production Version, Revision 2.1, dated Jun. 1, 1995, which is published by the PCI Special Interest Group of Hillsboro, Oreg.
The addressing mechanism of the PCI bus does not lend itself to the portable computer with expansion base unit design. As a standalone unit, the portable computer must handle the positive and subtractive decoding on the PCI bus. However, when coupled to the base unit, if subtractive decoding is maintained as was done when it was configured as a standalone unit, the PCI devices on the expansion base unit may conflict as both the PCI devices on the expanion base unit and the subtractive decoding device attempt to claim the PCI transaction. Furthermore, if the PCI address is delayed before it reaches the expansion base unit, the subtractive decoder device may claim the transaction before the expansion base unit is allowed to decode the address and thereby effectively lock out the devices on the expansion base unit. Therefore, a solution to this problem is desirable since coupling the expansion base unit to the portable computer with the PCI bus will provide a very high performance interface between the portable computer and the expansion base unit.